Revert "staging: comedi: drivers: Remove unneeded NULL check before kfree()"

This reverts commit 3545bee9.

It's not correct.

Reported-by: Hartley Sweeten <HartleyS@visionengravers.com>
Cc: Janani Ravichandran <janani.rvchndrn@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>